Although cystic fibrosis (CF) is the most common autosomal recessive genetic disorder of the white population, there are often delays in diagnosis and hence initiation of treatment. Advances of the past decade have made CF screening feasible using routinely collected neonatal blood specimens and determining immunoreactive trypsinogen (IRT) levels and CF mutations by DNA analyses. The overall goal is to address the hypothesis that early diagnosis of CF through neonatal screening will be medically beneficial without major risks. Medically Beneficial refers to better nutritional and/or pulmonary status, whereas major risks include missed diagnoses, potential iatrogenic medical sequelae, and adverse psychosocial consequences. Specific aims include assessment of the benefits, risks, costs, and quality of life associated with CF neonatal screening and the delineation of the characteristic epidemiologic features of CF. A comprehensive randomized clinical trial emphasizing early diagnosis as the key variable has been underway since 1985. Nutritional status has been assessed by anthropometric and biochemical methods, and the results have demonstrated significant benefits in the screened group. Answering the important question about a pulmonary outcome will require three more years of follow-up evaluation focused on lung function indices and quantitative chest radiology. The investigators note that if the questions underlying this study are answered favorably, it is likely that neonatal screening using a combination of IRT and DNA tests will become the routine method for identifying new cases of CF and that diagnosis in early infancy will allow prevention of many clinically significant problems such as malnutrition. They further note that if CF neonatal screening is implemented nationally, however, several epidemiologic gaps must be closed, including more precise data on the incidence and course of this disease and determination of risk factors for pulmonary infection. The state that this project will generate that important information.